The present disclosure relates to a technique for generating a driving current for a light source, which emits a light beam for forming an electrostatic latent image, on the basis of an analog signal generated by smoothing a periodic pulse signal indicating a light amount of the light beam.
Formation of an image by an electrophotographic system includes a step of forming an electrostatic latent image of an image indicated by image data on a photosensitive drum, a step of supplying a toner to the electrostatic latent image to form a toner image, a step of transferring the toner image onto a sheet, and a step of fixing the toner image, which is transferred onto the sheet, on the sheet.
In the step of forming an electrostatic latent image, an electrostatic latent image is formed on the photosensitive drum by repeatedly reflecting a light beam, which is emitted from a light source by subjecting the light source to light emission control, with a polygon mirror and rendering a main scanning line on the rotating photosensitive drum.
When the magnitude of a driving current for the light source is fixed and the main scanning line is rendered on the photosensitive drum, a light amount (in other words, intensity) of the light beam irradiated on the photosensitive drum is different according to a position on the photosensitive drum. As a cause of the difference in the light amount of the light beam, for example, the distance between the photosensitive drum and the polygon mirror is different in the center and at both the ends of the photosensitive drum (the distance between the polygon mirror and the center of the photosensitive drum is shorter than the distance between the polygon mirror and both the ends of the photosensitive drum). An optical characteristic of a condensing lens arranged between the polygon mirror and the photosensitive drum is also a cause of the difference in the light amount of the light beam.
When the light amount of the light beam irradiated on the photosensitive drum is different according to the position on the photosensitive drum, unevenness occurs in the density of an image.
Therefore, the magnitude of the driving current for the light source is adjusted during main scanning to fix, on the photosensitive drum, the light amount of the light beam irradiated on the photosensitive drum. For example, there is proposed a technique for dividing the main scanning line into a plurality of regions (blocks), generating a pulse width modulation (PWM) signal indicating, for each of the regions, a light amount of a light beam emitted by the light source, smoothing the PWM signal, generating an analog signal having magnitude corresponding to the light amount of the light beam, and adjusting the magnitude of the driving current for the light source on the basis of the analog signal.
When the PWM signal is smoothed to generate the analog signal, ripples occur in the analog signal according to the period of the PWM signal. The ripples sometimes cause deterioration in image quality.
When a circuit for smoothing the PWM signal is a low-pass filter consisting of a CR filter, if a time constant of the CR filter is increased, it is possible to prevent ripples from occurring in the analog signal. However, if the time constant is increased, a response time from the input of the PWM signal to the CR filter until the output of the analog signal increases, leading to a delay in control of the light amount of the light beam. Therefore, there is a limit in increasing the time constant of the CR filter.
It is an object of the present disclosure to provide an image forming apparatus that can prevent deterioration in image quality due to ripples that occur in an analog signal generated by smoothing a pulse signal and used for generating a driving current for a light source that emits a light beam for forming an electrostatic latent image.